In most running, walking, and jumping activities, the return force resulting from foot strikes causes great shock to the body. The stress from repeated foot strikes places great stress on joints and bones, and can cause injuries to the lower back and the rotating joints of the legs.
To minimize injury to the body resulting from repeated foot strikes, and also to improve athletic performance, shoe engineers have designed various spring-cushioned shoes. The springs in spring-cushioned shoes are designed to reduce shock to the body during a foot strike, and also to recover and return impact energy to the user. One type of spring-cushioned shoe is described in U.S. Pat. No. 6,282,814 to Krafsur et al., which is incorporated herein by reference. Two other types of spring-cushioned shoes are described in U.S. Pat. No. 5,743,028 to Lombardino, and U.S. Pat. No. 4,815,221 to Diaz. The Lombardino '028 patent discloses a plurality of vertical compression springs located in the heel area of a running shoe. The springs of the '028 patent are housed in a hermetically sealed unit filled with a pressurized gas which in combination with the springs provides a shock absorption and energy return system. The Diaz '221 patent discloses an energy control system placed within a cavity in the sole of a shoe. The energy control system includes a spring plate with a plurality of spring projections distributed over the surface of the plate for propulsion and shock absorption.